Spacecraft propulsion generally falls into two classes—chemical and electric. Chemical propulsion thrusters (solid motors, liquid engines, or hybrids) have high thrust, which translates to fast maneuverability and short transit times at the expense of using a relatively large mass of propellant. Electric propulsion (EP) thrusters have high exhaust velocity and low thrust. EP thrusters have the advantage of using about 1/10th the propellant mass at the expense of relatively long transit times. The EP systems also provide a smaller impulse-bit than the chemical propulsion systems making them better suited for fine positioning spacecraft maneuvers.
In general it would seem that a spacecraft designer would always choose the EP system to take advantage of the propellant mass savings, however, for many spacecraft maneuvers the longer trip times of the EP system are unacceptable. In general EP is used for maneuvers where trip time is not an issue. These include north-south station-keeping (NSSK) for geosynchronous (GEO) satellites, east-west station-keeping (EWSK), attitude control, and drag make-up in low earth orbits (LEO). In contrast, chemical systems are used in applications such as orbit transfer, divert propulsion on interceptor satellites used in missile defense, and proximity operations for microsatellites performing maneuvers near other space objects, where time is of an essence.
To overcome the inherent disadvantages of both propulsion systems, it is common in spacecraft design to include multiple propulsion systems to independently provide high thrust and high exhaust velocity capability. For example, a Boeing 702 communications satellite contains two independent propulsion systems; a chemical rocket is used for a rapid orbit transfer from low orbit to a position near the final geosynchronous orbit, while an EP system is used to complete the orbit transfer using less propellant mass than the chemical rocket would have used. Once in a geosynchronous orbit the EP thrusters move to face north and south. For the 15 year lifetime of the satellite they will be fired for about 1 hour/week to perform NSSK orbit corrections.
One prospective example of the need for multiple propulsion systems on satellites is for use on microsatellites. One microsatellite mission is to act as a defensive escort for a high-value space asset such as the International Space Station, or an expensive communications satellite. These escort satellites would need EP to fly near the host satellite while consuming a minimum of propellant, however, if a threat to the host appears, the escort microsatellite will need high thrust chemical propulsion to quickly respond and intercept the threat. Alternatively, if a microsatellite was designed for inspection missions the microsatellite would need EP for precision pointing and drag make-up, however, near the target the microsatellite will need high-thrust chemical propulsion to circumnavigate the object at close range.
Despite the clear need for a multiple mode propulsion system, such systems are are inherently complex because of the need for multiple control systems and fuel sources. There are currently no propulsion systems that can operate in a chemical or electric mode using the same propellant for the two modes of operation.
Accordingly, a need exists for a propulsion system having both high thrust and high specific impulse propulsion capabilities